A recently bought a bat detector, which makes the chirrups of bats
audible by modulating them against another high frequency.

It is, of course, effective at detecting bats. But it also opens up a
whole new ultrasonic world. A quartz watch whistles, a mechanical
watch becomes a rhythm-and-bell
orchestra, and a walk in the woods
becomes a cacophony of crushed leaves and previously unheard
animalogenic noises.

Most mobile telephones and many other gadgets have microphones
which will pick up ultrasonics quite well. I suggest an app for such
devices, to translate ultrasonics into the audio range, opening up a an
entirely new (if largely useless) world to, ah, the world.

Certainly there are - but I bet they aren't fitted as standard in mobile phones.

The fastest A-to-D converter I ever got to play with sampled at 40MHz. That's more than enough to reproduce a bat's sonar. It wouldn't have run for long from a phone battery though.

As for the modulation bit, that's easily done in software. It could be as simple as inverting the sign of every second block of N samples, followed by a simple low-pass filter. Something like a Nexus One has plenty of CPU grunt to do this kind of DSP in real time.

[Useless segway] What gets me is that I can hear bats. Click click as they fly around. I've told people and generally I get funny looks, but I can hear (at least part of) the sonar used by several bat species from where I live. [/UW]

...Anyhoo, can the ultrasonic transducers used for thickness testing and the like be used? How about the ultrasonic transducers used for stockpile height or feedbin/hopper level? Or maybe depth sounder/fish finder transducers? I suppose I just want this to work, sounds fun...

I'd tend to agree with [csea]'s observation, it wasn't long ago that mobile phones couldn't get any reasonable sample rate at all while recording.

Subsonic frequencies would be present though. So with a bit of filtering and up-shifting an elephant detector might be quite feasible, but before getting the soldering iron out I'll make some garlic bread as I've read its pretty good at flushing poisonous metals from the system.

I'm wondering if you'd be able to hear ultrasonic sound by playing a similar frequency sound and listening for the 'beats' between the two frequencies (see link) - e.g. if there's an ultrasonic sound at 25kHz, could I play a 20kHz tone and hear 'beats' at 5kHz? I think probably not, as the beat frequency is that of the modulated volume and you still wouldn't be able to hear the underlying frequencies, but I'm not sure.

//Bone until you can cite a reliable source for your
technology claims.//

Point taken, alas. I hereby bone myself. However, I
should point out that bats, at least, are bloody loud at 15-
30kHz, so a cellphone mike would probably pick up
enough.

//Click click as they fly around. I've told people and
generally I get funny looks//
A lot depends on the species of bat and what it's doing.
Most people pre-middle-age can hear at least some of the
noise made by many bat species. But what you can hear is
only a teeny fraction of what they're saying.

WP: "Bats emit calls from about 12 kHz to 160 kHz, but the upper frequencies in this range are rapidly absorbed in air and many bat detectors are limited to around 15 kHz to 125 kHz at best."

Given that electrets, especially the smaller mobile phone type go up to 20K then I guess this is feasible after all. Then you're limited to the sampling rate and modern phones can probably go up to the full 44kHz. If you search around your desktop you probably can't find anything that goes above 48kHz, and its unlikely that common software will support much more including mobile phone OS's etc.

To recap: the maximum recordable frequency on mobile phones is 22kHz so the low end of a bat call should be audible with suitable software. Specialised bat phones would needed to hear the whole thing.